The present invention concerns an improved broadband (about 1 octave) isolator structure operating in the VHF frequency range (200 to 500 MHz) at a high energy level (several tens of watts) and in a wide temperature range (-40.degree. to +70.degree. C) without substantial reduction of the figure of merit.
There has been described in U.S. Pat. No. 3,845,412 applied for by the Applicant on the Oct. 23, 1973 and entitled: "Wideband non reciprocal integrated circuits utilizing surface wave propagation", a broadband isolator structur utilizing surface waves of the quasi-TM mode or of the hybrid HE.sub.11 mode. FIGS. 3 and 4 of this patent illustrate forms of construction of an isolator whose wave adapter or launcher member, 53' and 66 respectively, has radiating slots 67 which favour the absorption, by a matched load 68, of the energy contained in the backward wave. The illustrated constructions operate in the 3 - 7 GHz range.
Designing isolators operating in the VHF band is a problem. It is well known from experience that gyromagnetic materials can only be used at frequencies of a value of more than 2/3 .gamma. M, where .gamma. is the gyromagnetic ratio and M is the saturation magnetization. Due to the saturation magnetization values of available materials, the range of this type of device cannot cover VHF.
Recent theoretical studies of the modes of propagation of electromagnetic waves in a thin slab of gyromagnetic material, as published for instance in the Proceedings on the Intermag Conference London 1974 by Messrs. COURTOIS, CHIRON and FORTERRE, under the title : "A survey of broadband stripline ferrite isolators" have shown the existence of two TE modes designated as the magnetostatic mode and the dynamic mode respectively.
The present invention concerns isolators utilizing the dynamic mode. Referring to the notations of FIG. 1, which illustrates a thin slab for gyromagnetic material associated with a trirectangular reference system O, x, y, z, subjected to the action of an external field H.sub.o along Oz, in which the energy travels along Oy, the dimension of the slab in this direction being assumed to be infinite, the dynamic mode is characterized by the following conditions :
______________________________________ Electric field Magnetic field ______________________________________ E.sub.x = O ##STR1## E.sub.y = O ##STR2## E.sub.z = A H.sub.z = O ______________________________________
where
A is a constant PA1 k.sub.x and k.sub.y are the wave numbers PA1 K and .mu. are the components of the permeability tensor .mu..sub.eff is the effective permeability of the material.